Main question is what can coexist with:
African Cichilds-other african cichlids, chinese algae eaters, syno catfish
South American Cichilds-there is a huge variety of them. dwarf species like rams, keyholes, apistos can work in community tanks. larger ones like firemouths, parrots, or severums usually are peaceful enough for a community tank of larger fish. large ones like oscars, jaguars, doviis, etc. or mean ones like convicts need to have tankmates chosen very carefully
Oscars- some of them will allow tankmates, and some will not. other oscars, severums, giant danios, mid-size plecos, large barbs, a single convict, maybe silver dollars. you need a big tank for one oscar, 75gallons at the minimum
Jack Dempseys- not sure, some can work in a larger semi-aggressive community, some will be fighters. electric blue jacks stay smaller and tend to be more peaceful
Red Devils- tend to be vicious. i would be very careful adding anything other than a pleco if you lke them and some larger dithers like giant danios
I also heard that South American and African cannot mix or that is a guarantee going to be a fish battle royal in the aqurium, is that true?
in a large enough tank people have done it, but african and american cichlids are different in many ways. africans like harder water, south americans prefer it soft. africans are herbivores, americans are carnivores. overstocking africans will reduce aggression, overstocking americans will get you a death tank.